The present invention is concerned generally with sealing glass compositions, in powdered form or admixed with a vehicle to form a sealing glass paste. More particularly, the present invention is concerned with a sealing glass composition that has excellent adhesion to seal edges, forms high strength devitrified seals, and is highly resistant to organic contamination. The composition is useful in sealing glass components such as cathode ray tubes including television picture tubes.
Crystallizable sealing glass compositions in powdered form are used to provide devitrified glass seals between the face plates (panels) and funnels of cathode ray tubes such as television picture tubes. Such sealing glasses have the property of melting and flowing at low temperatures, i.e., usually below 500.degree. C. and most frequently between about 425.degree. C. and 475.degree. C., which facilitates the wetting of the glass surfaces to be sealed. Sealing glasses (sometimes called "solder" glasses) customarily are thermally devitrifiable or thermally crystallizable in nature. The devitrified or crystallized glass has a melting point higher than the fiber softening point of the original sealing glass.
PbO-containing sealing glasses, and particularly PbO/B.sub.2 O.sub.3 /ZnO sealing glasses, are commonly used as base glasses in commercial sealing glass compositions. Other materials typically are added to the base glass to adjust its properties.
Sealing glasses usually are applied in paste form. The paste is made by adding a vehicle to the sealing glass powder in an amount sufficient to form an extruded ribbon or bead that maintains its shape while the paste is applied to the funnel and the funnel and face plate are sealed. The paste should be reasonably stable for at least three to four hours or more so paste can be applied successively to a large number of funnels to make commercial quantities of television bulbs. The vehicle constituents are pyrolyzable when they are subjected to a temperature below the temperature at which the sealing glass frit is fired such that they leave only an inappreciable amount, if any, of residue in the fired frit.
The face plate and funnel undergo several processing steps before they are sealed together. For example, green, blue and red phosphors are separately and successively applied to the face plate by known techniques such that the phosphors are present as multiple dots or stripes in an ordered arrangement on the inner surface of the face plate. Sometimes a carbon or graphite background is applied to the inner surface of the face plate surrounding the phosphors to provide a sharp contrast to the phosphors. The inner surface of the face plate is then aluminized, i.e., a thin aluminum film is deposited, so that an electrically conductive surface is formed. This aluminized surface is connected to a metal stud on the inner surface of the face plate. Several organic compounds usually are applied to the inner surface of the face plate during phosphor application and aluminization.
After the preparatory face plate and funnel processing steps have been completed, the sealing glass is applied to the seal edge of the funnel. The funnel and face plate are then joined, sealed in a "frit seal" thermal cycle, and cooled. The frit seal thermal cycle typically involves firing in a nonreducing atmosphere at a temperature sufficiently elevated to fuse the sealing glass (generally about 425.degree. C. to 475.degree. C.) and form a strong, adherent hermetic bond of devitrified sealing glass between the face plate and funnel.
After the face plate has been sealed to the funnel, the interior confines of the tube are evacuated by applying a vacuum thereto. The tube must be heated to a temperature of about 280.degree. C. to 410.degree. C. while being evacuated to assure that all volatile substances, such as moisture, adsorbed gases, and organic materials, are liberated and withdrawn from the interior surfaces and confines of the tube. The application of heat to the tube during exhausting inevitably results in some relational shifting of the face plate, funnel and solder glass seal. A strong devitrified seal is necessary to withstand the creation or concentration of physical stresses in the vicinity of the seal resulting from the relational shifting of parts during the heating operation and subsequent cooling.
The PbO in the sealing glass has a natural tendency to be reduced to metallic lead during frit sealing in a reducing atmosphere or in the presence of organic vapors. Seals in which the PbO has been reduced are gray or gray-black in color, indicating the presence of metallic lead, rather than the yellow color that is generally characteristic of devitrified PbO glass. The reduction of PbO hinders adhesion of the sealing glass to the seal edges and induces dielectric breakdown in the resultant seal when the seal is exposed to high voltage conditions such as those existing within a color television tube during its operation. Because the high voltages present in a television tube during its operation in a television set range from about 25 kV to 45 kV or more for a color television tube, any dielectric breakdown in the seal between the funnel and face plate will provide a source of tube malfunctions. A tube with appreciable amounts of metallic lead in its seal is unacceptable for use and is likely to be rejected when the tube undergoes a standard voltage test conducted at the tube manufacturing plant.
Various techniques may be used to prevent or limit reduction of PbO during firing of the glass seal. Some tube manufacturers volatilize or otherwise remove organic contaminants from the face plate before firing of the glass seal. This generally involves a prior heating cycle known as "panel bake." Other tube manufactures use sealing glasses with improved resistance to organic contamination such as those including red lead (Pb.sub.3 O.sub.4) or another oxidizing agent. The oxidizing agent is preferentially reduced to a lower oxide of the cation in the presence of reducing conditions.
Sealing glasses must have a suitable combination of properties to perform satisfactorily in television picture tubes and the like. For example, the sealing glass must have good stability when mixed as a paste and good tolerance of temperature deviations during the thermal soak time. The sealing glass also must adhere well to the seal edges of the funnels and face plates, form high strength crystallized seals, and resist organic contamination introduced during tube making. The resultant seals must have appropriate thermal expansion characteristics to avoid damage to the tube components during subsequent processing and appropriate dielectric characteristics to prevent failure of the tube when it is exposed to high voltages during use.
Sealing glass developers generally strive to achieve a sealing glass having a satisfactory balance of properties, but it is difficult to achieve this balance. When a sealing glass composition is modified to enhance performance in one area, performance in another area usually becomes less than ideal.
For example, the rate of crystallization and the size and distribution of crystals are important to formation of a strong seal. The ideal crystallization rate is fast but delayed enough to allow adequate chemical bonding of the prepared sealing glass to the face plate and funnel seal edges. Formation of large, fast-growing crystals within the glassy matrix results in an intrinsically strong seal. Crystals that form an interlocking structure rather than flower-like forms are preferred because they provide greater resistance to crack propagation.
U.S. Letters Pat. No. 4,589,899 to Hudecek (the "Hudecek patent"), incorporated herein by reference, discloses a crystallizable PbO/ZnO/B.sub.2 O.sub.3 /SiO.sub.2 /BaO sealing composition in powdered form that has superior crystallization properties. The addition of finely divided zinc zirconium silicate as a nucleating agent enhances the ability of the composition to form strong crystallized seals. It also improves the adhesion of the composition to seal edges over prior art sealing glasses. However, the composition provides only average resistance to organic contamination. A higher resistance to organic contamination would be beneficial in no-panel-bake manufacturing processes (processes in which the screened panel is not given a preliminary pre-bake to drive off organic contamination before frit sealing).
Similarly, U.S. Letters Pat. No. 3,973,975 to Francel et al. (the "Francel patent"), incorporated herein by reference, describes PbO-containing sealing glasses that include certain oxidizing agents in amounts from at least 0.1 to about 1.5% by weight of the sealing glass to control PbO reduction. Sealing glasses such as those taught by Francel are resistant to organic contamination but have not achieved commercial success because of poor flow, excessive porosity and the presence of oxygen bubbles. In addition, when Pb.sub.3 O.sub.4 is used as an oxidizing agent, the Pb.sub.3 O.sub.4 acts as a nucleation site for small slow-growing crystals that interfere with growth of a preferred larger crystal and extend the time needed to complete the crystallization to 90 percent or more crystallized. The presence of excess oxidizing agent also interferes with oxidation of the metal getter provided to improve the tube vacuum, reducing the life and effectiveness of the getter.
It is an object of the invention to provide a crystallizable sealing glass in powdered form that has superior characteristics, namely, good stability when mixed as a paste, good tolerance of temperature deviations during the thermal soak time, excellent adhesion to the seal edges, good resistance to organic contamination and the ability to form high strength crystallized seals. The sealing glass is useful in sealing cathode ray tube bulbs, particularly television picture tube bulbs.
It also is an object of the invention to provide a PbO-based sealing glass that resists the chemical reduction of PbO to metallic lead when the sealing glass is exposed to reducing conditions during sealing, particularly in no-panel-bake tube making operations, to produce a seal having suitable dielectric and other properties without the disadvantages associated with prior art sealing glasses.
These and other objects of the present invention will be apparent from the specification that follows, the appended claims, and the drawings.